Changing .avi file in to .mp4 file
This is off the Blender topic and rather belongs to https://video.stackexchange.com

How you want to use your video should drive what format and quality you need.

If you are comfortable with command line tools, I suggest ffmpeg  (https://www.ffmpeg.org). The dead simple answer is ffmpeg -i "input.avi" "output.mp4" though this will reencode it and you will loose some quality. If the video stream inside your AVI is h.264 or other MP4 compatible encoding, you can rewrap without quality loss using ffmpeg -i "input.avi" -c:v copy -c:a copy "output.mp4".